1. Field of the Invention
The present invention relates to an apparatus for optically detecting light-absorbing contamination in particles of low optical-loss material, such as polymer powder, beads, or pellets.
2. Description of the Related Art
Engineering materials, such as polymers, ceramics and filler materials used in manufacturing, are sold in particulate intermediate form as pellets before processing into final products. Also, many food products for human and animal consumption are produced in particulate form, pellets, grains and small vegetables being common examples. Medical products such as aspirin tablets, powders and similar materials are also manufactured in intermediate particulate form.
During manufacture and handling of materials in particulate or pellet form, it is possible for various types of contaminants to become mixed in with the material. These contaminants can be in the form of a pellet themselves or mixed in the desirable pellets. In either case, the result is the same--a degradation of the finished product. Ensuring that such particulate materials, in their intermediate form, are free of contaminants is an important quality control consideration.
The problem described above is so widespread in industrial production that many attempts have been made to automate the inspection of particulate material, with varying degrees of success. A number of factors must be considered when developing a system which can inspect particulate material for contamination, including materials handling considerations and the interaction between product handling and optical detection of contamination.
One approach to overcoming the problem described above has been to use a source of illumination, such as an arc lamp of suitable spectral content, and a detector, such as a photomultiplier tube or a PIN diode detector, with the necessary electronic circuitry to detect changes in light flux reflected from or transmitted through the particulate material. The particulate material is passed through the zone of illumination. A measurement in the form of an electrical signal proportional to the light flux provides the information to make a quality judgment. The contaminated particulate material is thus detected. The contaminant occupies a small percentage of the field of view of the detector and causes a relatively small percentage change in light flux. This approach forces the system designer to deal with signals that have a relatively low signal-to-noise ratio. The low signal-to-noise ratio problem usually causes an increase in system complexity and cost and adversely effects the performance of the system. In addition to signal-to-noise problems, the light intensity required in this system to obtain a sufficient contaminant signal amplitude may result in damage to the product due to heating or other light-induced change.
Another approach to optically detecting contamination in particulate material has been to use an illumination concept which is similar to that used in the above-described system with an array of detectors, such as a linear or a two-dimensional charge-coupled device (CCD) array. This approach images small areas of the field of view onto individual photosites and improves the signal-to-noise ratio. However, this approach is subject to signal degradation due to light scattering in the zone of illumination, flare in the imaging optics and crosstalk between photosites in the detector array itself. In addition, this technique is sensitive to the shape and orientation of the particles which may cause specular reflections. These specular reflections can cause large changes in signal level which are not related to the presence of contamination. Complexities result from data handling and analysis, due to timing and data format requirements of the detectors, which result in increased complexity and cost for this type of optical detection system.
It has been observed that under ordinary lighting conditions, a low optical-loss material such as a transparent object can be readily seen because of light intensity variations due to the direction of the illuminating light. It has been shown however, that such low optical-loss materials become nearly invisible when viewed in a perfectly uniform illumination field (see R. W. Wood, Physical Optics, third ed., The Macmillan Company, New York, (1934), p. 98). A lossless or transparent object may be placed within an optical integrating chamber and illuminated with light which is of equal intensity in all directions. If a person were to observe the object through a small opening in the integrating chamber, the object would be nearly invisible.